Compound solar cells which use CuInSe2 (CIS) composed of elements from Groups Ib, IIIb, and VIb or Cu(In,Ga)Se2 (CIGS) compound semiconductors with Ga therein present as solid solutions (Group I-III-VI compound semiconductors) for light absorbing layers are known to be high in light conversion efficiency (hereinafter, referred to as “conversion efficiency”), able to be formed into thin films, and advantageous in that the conversion efficiency is less likely to be degraded due to light irradiation or the like.
For a buffer layer of a CIGS solar cell using such a CIS or CIGS (hereinafter, referred to as “CIGS”) compound semiconductor for a light absorbing layer, CdS, Zn(O,S), or the like formed by a chemical deposition method is typically used (for example, see Patent Document 1). However, in the case of forming the buffer layer by the chemical deposition method, it is necessary to form a CIGS compound semiconductor layer by a vapor deposition or selenization method under vacuum, then once extract the layer under the atmosphere, form the buffer layer, and form a transparent electrode layer again under vacuum, and there is the problem of poor productivity.
Therefore, in order to solve this problem, it has been proposed that the buffer layer is formed continuously by a sputtering method under vacuum without the extraction under the atmosphere (for example, see Patent Document 2).